1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.
2. Description of the Related Art
As a fixing device mounted in an image forming apparatus such as an electrophotographic printer or an electrophotographic copying machine, there exists a film heating type fixing device which has a heater having a heat generating resistor on a ceramic substrate, a fixing film configured to move while held in contact with this heater, and a pressurization member configured to be pressurized with a predetermined pressurization force to thereby form a nip portion together with the heater via the fixing film. In the film heating type fixing device, while a recording material bearing an unfixed toner image is being pinched and conveyed by the nip portion, the toner image is fixed to the recording material through heating.
In an image forming apparatus in which such a fixing device is mounted, a mechanism for changing the pressurization state of the fixing film and the pressurization member (hereinafter referred to as the pressure changing mechanism) is provided as needed. This pressure changing mechanism can make the pressurization force between the fixing film and the pressurization member other than at the time of fixing lower than that at the time of fixing. The reason for providing this mechanism is to prevent setting of an elastic member such as rubber used in the fixing film and the pressurization member; such setting will occur if the fixing film and the pressurization member are kept at rest while in the pressurization state.
A fixing device provided with such a pressure changing mechanism is discussed in Japanese Patent Application Laid-Open No. 2007-256875.
Conventionally, in an image forming apparatus in which a fixing device provided with a pressure changing mechanism as discussed in Japanese Patent Application Laid-Open No. 2007-256875 is mounted, electricity supply to the heater is started after the fixing film and the pressurization member have been placed in the pressurization state, in which fixing is feasible. The reason for doing this is to prevent cracking of the heater; for, when electricity is supplied to the heater in a state in which the pressure is lower than that in the pressurization state, in which fixing is feasible, the temperature of the heater rises abruptly, resulting in cracking of the heater. In this configuration, however, the requisite time for the fixing device to attain a predetermined temperature at which fixing is feasible when starting printing (hereinafter, this requisite time will be referred to as the warm-up time) is longer by the operation time of the pressure changing mechanism, so that it is rather difficult to shorten the first print out time (hereinafter referred to as the FPOT). The FPOT is the time it takes for the first sheet to be discharged after the input of a printing start signal; it is important to shorten this FPOT from the viewpoint of usability. Thus, an image forming apparatus with a fixing device equipped with a pressure changing mechanism, which is free from cracking of the heater and capable of further shortening the warm-up time, is demanded.